1. Field of the Invention
The present invention relates to an apparatus and method for welding pipes together. More specifically the invention relates to arc-welding together pipe sections when laying pipelines underwater, especially at sea.
2. Description of the Related Art
When laying a pipeline at sea it is customary to weld, on a lay-barge, individual pipe sections to a pipe string (the pipe string leading towards the seabed). The welding process takes place on the lay-barge. The pipe sections may consist of a plurality of pipe lengths each welded together on the lay-barge to form the pipe sections when required.
The pipe-string, when being laid, is under great tension and weld joints must, of course, necessarily be sufficiently strong to withstand the high forces imposed on the weld joints. Each time a pipe is welded to another pipe extensive tests are made to ensure that the quality of the weld joint formed is sufficient. The strength of a weld joint depends upon various factors, one being the geometry of the path traced by the point of contact of the arc in relation to the surfaces of the pipes to be joined. If the point of contact of the arc is off target by as little as a tenth of a millimeter the quality of the joint may be reduced by enough that the pipe joint is rejected, when tested, as not being of sufficient quality. It is therefore important that the weld metal is laid down in the region of the joint with great accuracy.
Furthermore the radial distance of the electrode with respect to the pipes must change in relation to the depth of the weld joint. As the region of the joint between the pipes is filled with welded metal the surface of the welded metal gets closer to the welding torch.
There are therefore special considerations that must be taken into account when designing an apparatus for welding such pipes together.
A known method of welding two pipes together may be described as follows. The pipes to be joined are prepared prior to the welding process by bevelling the ends of the pipes such that when the pipes are arranged immediately before the welding process commences (coaxially with respect to each other), an exterior circumferential groove is defined between the two pipes. The pipes are positioned ready for welding. A carriage is mounted on one of the pipes for movement around the circumference of the pipes to be joined. A welding torch is mounted on the carriage and the apparatus is so arranged that the end of the metal electrode of the torch is opposite and relatively close to the circumferential groove. The carriage is moved around the circumference of the pipe and the torch is operated so that an arc is directed into the groove. The arc is guided manually and/or by various mechanical sensors to guide the arc as accurately as possible along the length of the groove. The welding process generally takes several passes. In the above-described method the resolution of the mechanical sensors is/such that a human operator is required to assist in the welding process for guiding the arc with sufficient accuracy.
The time it takes to lay a given length of pipeline is, to a great extent, determined by the time it takes to perform all the necessary welding operations. There is therefore a benefit in reducing the time it takes to weld two pipes together. Any attempt to speed up the welding process should not however lead to a significant reduction in the quality of the weld joint.